Based on the principle of ultrasonic transmission method with some hypothesis, a relationship model between porosity, the number of layers and the ultrasonic attenuation was established for different CFRP laminates. Firstly, under several assumed conditions, a relationship model between the number of layers and the ultrasonic attenuation was established without porosity. This model can reflect effect of other factors on the ultrasonic attenuation. Then, considering the effect of voids, the model was improved with the factor of porosity. Finally, using the test data of the porosity and ultrasonic attenuation for 5 different laminate test blocks, a fitting porosity mode was given. The analysis and test result show the model has enough precision and can certify the requirement of engineering test. This model can be used to test the porosity of different composite laminates in same bath and doesnt need to prepare too many samples, so it is convenient and fast, as well as cost saving. On the basis, a relationship model between porosity, thickness and ultrasonic attenuation was established by revising the porosity model. As for composites with variable thickness, it is needed to replace the number of layers by thickness. Whats more, refraction phenomenon may occur while the ultrasound propagates at the back wall. So other factors should be revised. The analysis shows that, the included angle between the propagation direction and the normal direction of back wall plays an important part in the model. And the attention will increase rapidly with the angle increases. So the model is suitable for the composites with big curvature radiuses.